Tyrosine phosphorylation of synaptic receptors and signaling molecules regulates synaptic activity. A number of protein tyrosine phosphatases specifically expressed within the brain have been identified, including STEP (for STriatal-Enriched tyrosine Phosphatase, also known as PTPN5). Recent evidence suggests that STEP plays an important role in synaptic plasticity, for review see (Braithwaite S P. et al., (2006), Trends Neurosci, 29 (8): 452; Baum M L, et al., (2010), Commun Integr Biol, 3 (5): 419) STEP is specifically expressed within neurons of the central nervous system. As its name indicates, the highest expression level is within the striatum. However, more recent work has found that it is expressed at lower levels in multiple brain regions including the neocortex, amygdala, hippocampus, and embryonic spinal cord.
Four groups of proteins that STEP regulates have been identified: the mitogen-activated protein kinases (MAPKs), the tyrosine kinase Fyn, the N-methyl-D-aspartate (NMDA) receptor complex (specifically the NR2B subunit) and AMPA receptors (specifically, GluR2, (Zhang Y, et al., (2008), J Neurosci, 28 (42): 10561)). Three additional new substrates for STEP have also been recently discovered; proline-rich tyrosine kinase 2 (Pyk2; Xu J, et al., (2010), Abstracts of the Society for Neuroscience Meetings), the fragile X mental retardation protein (FMRP) (Goebel-Goody S M, et al., (2010), Abstracts of the Society for Neuroscience Meetings) and the cell-death mediator Bak (Fox J L, et al., (2010), EMBO J, 29 (22): 3853). Tyrosine phosphorylation of one member of the MAPK family, the extracellular signal regulated kinase (ERK), is necessary for the expression and maintenance of synaptic plasticity in many brain regions, and disruption of the ERK pathway leads to a disruption of learning and memory. One of the functions of these src and Pyk2 kinases is to phosphorylate NMDA receptors, thereby modulating their channel conductance properties and facilitating their movement toward the surface of neuronal plasma membranes. Pyk2 and Fyn tyrosine kinases are activated by phosphorylation on tyrosine residues. NR2B phosphorylation on Tyrosine 1452 inhibits the receptor endocytosis. STEP acts as direct or indirect brake of NMDAR mediated signaling by either respectively dephosphorylating NR2B or its associated kinases, Pyk2 and Fyn. Activation of AMPA, NMDA receptors and MAPKs are required for the induction of several forms of long-term potentiation (LTP) and long-term depression (LTD). Hippocampal LTP is increased in transgenic mice model of Alzheimer lacking STEP (Zhang Y, et al., (2010), Proc Natl Acad Sci USA, 107 (44): 19014). NR2B and AMPA receptor surface expression is increased in STEP KO mice. AMPA receptor endocytosis in group I metabotropic glutamate receptor I (mGluR) mediated LTD is mediated by a tyrosine phosphatase. AMPA receptor endocytosis induced by activation of group I mGLuR is blocked in STEP KO mice suggesting that STEP might also control mGluR mediated LTD.
Compounds that inhibit STEP activity should mimic the effects observed with the STEP KO and may be useful for treating conditions mediated by abnormal NMDA-receptor (NMDA-Rs) and/or MAP kinase pathway signaling. Both may mediate cognition, learning and memory, neurogenesis, and may also affect neuronal plasticity, pain perception, mood and anxiety, and neuroendocrine regulation.
Modulation of NMDA-Rs:
STEP decreases the tyrosine phosphorylation level of NMDA-Rs. Less phosphorylated NMDA-Rs have lower conductance states and thus will allow less current and fewer ions to pass. The NMDA-Rs will therefore be functionally less active (Alvestad K M, et al., (2003), J Biol Chem, 278 (13): 11020), which can lead to schizophrenic symptoms. Hypofunction of NMDA-Rs has been liked to schizophrenia. For example, phencyclidine, ketamine, and other noncompetitive antagonists at NMDA-type glutamate receptors can exacerbate symptoms in patients (Lahti A C, et al., (1995), Neuropsychopharmacology, 13 (1): 9) and may produce a range of psychotic symptoms in volunteers that are similar to those of schizophrenic patients. NMDA-R hypofunction is also linked to psychosis and drug addiction (Javitt D C and Zukin S R, (1991), Am J Psychiatry, 148 (10): 1301). Chronic treatment of atypical antipsychotic clozapine and risperidone in mice result in significant increase of phosphorylation of ERK, NR2B and Pyk2 on tyrosine residues recognized by STEP (Carry N C, et al., (2010), Abstracts of the Society for Neuroscience Meetings). Treatment of these anti-psychotics also enhances cAMP and STEP phosphorylation. Since PKA mediated phosphorylation of STEP is know to inactivate STEP, these results suggest that STEP inhibition mediates the beneficial effect of antipsychotic drugs. Recent studies have linked abnormal NMDA-R activity and expression of STEP to the cognitive decline observed in Alzheimer's disease or transgenic mice expressing mutant APP (Tg2576 mice) (Snyder E M, et al., (2005), Nat Neurosci, 8 (8): 1051; Hynd M R, et al., (2004), J Neurochem, 90 (4): 913; Kurup P, et al., (2010), Channels (Austin), 4 (5)). More specifically, STEP KO mice are less susceptible to PCP-induced hyperlocomotion and PCP-induced cognitive deficits in the object recognition tasks (Carty N C, et al., (2010), Abstracts of the Society for Neuroscience Meetings). Compared to the Tg2576 mice expressing STEP, Tg2576 lacking STEP gene showed rescue in their deficits in hyppocampal LTP and in different behavioral cognitive tasks. Altogether, these results suggest that STEP inhibitors might represent a novel class of drugs that can treat both positive symptoms and cognitive deficit associated with schizophrenia.
Medications that modulate glutamatergic neurotransmission via NMDA-Rs may be also effective in treatment for mood and anxiety disorders. Administration of NMDA-R antagonists has anxiolytic effects in rodent models of anxiety (Falls W A, et al., (1992), J Neurosci, 12 (3): 854; Miserendino M J, et al., (1990), Nature, 345 (6277): 716). NMDA-Rs antagonist like ketamine has been shown to be effective in drug-resistant unipolar depression (Machado-Vieira R, et al., (2009), Pharmacol Ther, 123 (2): 143).
Abnormal balance between the activity of NMDA receptors at synaptic (prosurvival linked to ERK activation) and extrasynaptic (proapoptotic linked to p38 activation) sites has been proposed in cellular and mouse model of Huntington Disease (HD) (Milnerwood A J, et al., Neuron, 65 (2): 178). YAC128 mouse model (containing high number of glutanmine repeat on huntingtin) of HD showed an increased activity of extrasynaptic NMDA receptors (NR2B subunit) and require p38 and caspase-6 cleavage activation. In YAC128 mice, NR2B synaptic expression is associated with high STEP expression and activity and a reduction in NR2B expression and phosphorylation (Gladding C M, et al., (2010), Abstracts of the Society for Neuroscience Meetings). Extrasynaptic NMDA receptors couple preferentially to excitotoxicity via calpain-mediated cleavage of STEP and activation of p38 (Xu J, et al., (2009), J Neurosci, 29 (29): 9330). Inhibiting STEP activity might therefore shift the balance toward the NMDA receptor/ERK synaptic prosurvival signaling pathway.
Modulation of ERK Pathway:
STEP inhibition may translate into activation of ERK1/2 kinases, for example, in the central nervous system (CNS). Activation of the ERK pathway in the CNS can mediate neurotrophic pathways involved in cellular resilience. ERK signaling directly affects Bak phosphorylation through inhibition of STEP to promote cell survival (Fox J L, et al., (2010), EMBO J, 29 (22): 3853). BDNF and other neurotrophins can block apoptosis and increase cell survival of different type of CNS neurons in vitro and in vivo via stimulation of the ERK pathway. Mood stabilizers effective in bipolar disorder like valproate and lithium may be potent activators of ERK activity. This effect on ERK activation is believed to be responsible for the neurotrophic effects of mood stabilizers observed in vitro or in brains of treated patients with bipolar disorder, for review see (Engel S R, et al., (2009), Mol Psychiatry, 14 (4): 448; Chen G and Manji H K, (2006), Curr Opin Psychiatry, 19 (3): 313; Machado-Vieira R, et al., (2009), Bipolar Disord, 11 Suppl 2 92). In vivo disruption of STEP activity was shown to activate MAPK pathway, leading to significant rescue from neuronal cell death after pilocarpine-induced status epilepticus (Choi Y S, et al., (2007), J Neurosci, 27 (11): 2999). Increasing cellular resilience could therefore limit or reduce neuronal loss in several neurologic disorders. Recent work has suggested a positive role for STEP inhibition in fragile X syndrome (FXS). This disorder results from the mutation of fmr1 gene coding for the fragile X mental retardation protein (FMRP). STEP binds to FMRP and its expression is dysregulated in FXS. FMR KO mice model displayed audiogenic seizures. FMR KO mice lacking STEP gene show a significant reduction of these seizures (Goebel-Goody S M, et al., (2010), Abstracts of the Society for Neuroscience Meetings), suggesting that STEP modulators might be therapeutic approach for FXS.
Various substituted heterocyclic compounds are disclosed in the art. For example, WO 02/062767 discloses quinazoline derivatives; WO 03/000188 discloses quinazolines and uses thereof; WO 2005/042501 discloses norepinephrine reuptake inhibitors for the treatment of central nervous system disorders; WO2006/058201 discloses heterocyclic and bicyclic compounds, compositions and methods; WO 2007/104560 discloses substituted 4-amino-quinazoline derivatives as regulators of metabotropic glutamate receptors and their use for producing drugs; WO 2007/133773 discloses CDKI pathway inhibitors; WO 2008/009078 discloses 4,6-DL- and 2,4,6-trisubstituted quinazoline derivatives useful for treating viral infections; WO 2009/000085 discloses quinoline and quinazoline derivatives useful as modulators of gated ion channels; US 2009/0143399 discloses protein kinase inhibitors; and Japan Publication Number 2007-084494A discloses substituted bicyclic compounds.